FIG. 7 is a view showing an interference exposure apparatus used for manufacturing a conventional diffraction grating. Referring to FIG. 7, a laser beam 17a is output from a laser beam source 4. The laser beam 17a is divided into laser beams 17b and 17c by a half mirror 5 and the laser beams 17b and 17c are reflected by mirrors 6a and 6b, respectively and then applied to a resist 2 on a substrate 1.
FIGS. 8(a), 8(b), 8(d), and 8(e) are cross-sectional views showing conventional manufacturing steps of a diffraction grating by an interference exposure method and FIG. 8(c) is a diagram showing a distribution of exposure intensity.
Next, the conventional manufacturing steps of the diffraction grating will be described in reference to FIGS. 8(a)-8(e).
First, referring to FIG. 8(a), a resist 92 is applied to a substrate 91. Then, referring to FIG. 8(b), the resist 92 is exposed by a two-beam interference exposure method. Referring to FIG. 8(c), the exposure intensity of a laser beam applied to the resist 92 varies periodically. Then, referring to FIG. 8(d), when the exposed resist 92 is developed, the resist 92 is patterned and a diffraction grating 93a is formed. Thereafter, referring to FIG. 8(e), the substrate 91 is etched away using the patterned resist 92 as a mask and then a diffraction grating 93b is formed.
Then, a principle of the interference exposure method used for manufacturing the conventional diffraction grating will be described in reference to FIG. 7.
According to the device shown in FIG. 7, the laser beam 17a output from the laser beam source 4 is divided into two by the half mirror 5 and reflected by the mirrors 6a and 6b and meet again on the substrate 1. At this time, intensity of the beam on the substrate has the distribution of a period .LAMBDA. because of interference of the laser beam. The period .LAMBDA. is represented by: ##EQU1## where .lambda. is a wavelength of the laser beam and .theta. is the incident angle of the laser beam on the substrate.
The conventional diffraction grating is formed using the above principle, in which the resist applied to the substrate is exposed with a period .LAMBDA. and then the resist is developed and then the substrate is etched away using the patterned resist as a mask.
FIG. 9 is a sectional view showing a conventional single wavelength semiconductor laser device disclosed in Optics Vol. 15, No 2, pp.115-121. In FIG. 9, an n type InGaAsP guide layer 107, an InGaAsP active layer 108 and a p type InP layer 109 are sequentially formed on an n type InP substrate 101 in which a diffraction grating 102 having a phase shift region in the center thereof is formed. An n side electrode 110 is provided on a back surface of the substrate 101 and a p side electrode 111 is provided on the p type InP layer 109. In addition, a nonreflective coating film 113 is provided on each end surface 114 of the laser.
Next, operation thereof will be described. In the semiconductor laser device, electrons in the n type InP substrate 101 and holes in the p type InP layer 109 are both injected into the InGaAsP active layer 108 and then emissive recombination occurs. In a distributed feedback (DFB) laser device having the diffraction grating 102 having the phase shift region in an active region, the light generated by the emissive recombination is reflected by the diffraction grating 102 and goes and returns in an element, whereby a laser is oscillated.
Since the diffraction grating 102 effectively reflects a beam having a wavelength .lambda. where .lambda.=2 n.sub.eff .LAMBDA./n (n.sub.eff is equivalent refractive index, .LAMBDA. is the period of the diffraction grating and n is an integer), the oscillation wavelength is the wavelength whose gain is largest in the active region among wavelengths represented by 2 n.sub.eff .lambda./n. As for the oscillation wavelength .lambda., when n=1, the diffraction grating is called a primary diffraction grating and when n=2, the diffraction grating is called a secondary diffraction grating.
According to the single wavelength oscillating semiconductor laser device having the structure shown in FIG. 9, since the beam is reflected only by the diffraction grating 102 in the active region and then confined in the element, beam density in the center of the element is increased. As a result, the linearity between beam output and an injected current and the stability of single wavelength oscillation are reduced by hole burning or the like.
As described above, according to the conventional method for manufacturing a diffraction grating, it is not possible to form a diffraction grating having a period which is a half of the wavelength .lambda. of the laser beam source 4 or less.
In addition, according to the conventional single wavelength oscillating semiconductor laser, the linearity of the beam output relative to injected current and stability of the single wavelength oscillation is reduced under an influence of the hole burning or the like.